A bipolar junction transistor (BJT) is a three-terminal device that can controllably vary the magnitude of the electrical current that flows between two of the terminals. The three terminals include a base terminal, a collector terminal, and an emitter terminal. The movement of electrical charge carriers, which produce electrical current flow between the collector and the emitter terminals, varies dependent upon variations in the voltage on the base terminal thereby causing the magnitude of the current to vary. Thus, the current flow through the emitter and collector terminals is controlled by the voltage across the base and emitter terminals. The terminals of a BJT are connected to their respective base, collector, and emitter structures formed in a semiconductor substrate. BJTs comprise two p-n junctions placed back-to-back in close proximity to each other, with one of the regions common to both junctions. There is a first junction between the base and the emitter, and a second junction between the emitter and the collector. This forms either a p-n-p or n-p-n transistor depending upon the characteristics of the semiconductive materials used to form the HBT.
Recently, demand for BJTs has increased significantly because these transistors are capable of operating at higher speeds and driving more current. These characteristics are important for high-speed, high-frequency communication networks such as those required by cell phones and computers.
BJTs can be used to provide linear voltage and current amplification because small variations of the voltage between the base and emitter terminals, and hence the base current, result in large variations of the current and voltage output at the collector terminal. The transistor can also be used as a switch in digital logic and power switching applications. Such BJTs find application in analog and digital circuits and integrated circuits at all frequencies from audio to radio frequency.
Heterojunction bipolar transistors (HBTs) are BJTs where the emitter-base junction is formed from two different semiconductive materials having similar characteristics. One material used in forming the base-emitter junction preferably is a compound semiconductive material such as silicon-germanium (SiGe) or silicon-germanium-carbon (SiGeC). HBTs using compound semiconductive materials have risen in popularity due to their high-speed and low electrical noise capabilities, coupled with the ability to manufacture them using processing capabilities used in the manufacture of silicon BJTs. HBTs have found use in higher-frequency applications such as cell phones, optical fiber, and other high-frequency applications requiring faster switching transistors, such as satellite communication devices.
Most BJTs, including HBTs, in use today are “double poly” bipolar transistors, which use two polysilicon structures; one for an emitter structure, and a second for a base structure of the transistor.
HBTs are manufactured by implanting a silicon substrate with a dopant to provide a collector region. A silicon layer is then grown or deposited over the collector region. Insulating dividers called shallow-trench isolations (STIs) are formed in the silicon substrate. The STIs define an intrinsic base region over a portion of the collector region.
Subsequently, a first layer of polysilicon is deposited over the silicon substrate and is processed to form a base structure in contact with a portion of the intrinsic base region. One portion of the base structure is formed with an opening in which an emitter structure is subsequently formed.
A first insulating layer is deposited over the base structure and is removed in the opening of the base structure over the intrinsic base region by etching down to the intrinsic base region to form an emitter window. The etching process inherently produces a rough surface on the substrate since the etchants used are not particularly selective between the polysilicon layer forming the base structure and the underlying silicon substrate.
To get higher performance, compound semiconductive materials such as SiGe and SiGeC generally are grown over the insulating layer and on the rough surface of the substrate. The rough surface causes a major problem because the growth of the compound semiconductive material is irregular and its thickness is not constant as a result of the roughness of the substrate. This leads to performance problems with the device and variations in performance from device to device. It also causes junction leakage and more power consumption.
A second layer of polysilicon is deposited into the emitter window over the compound semiconductive materials and processed to form an emitter structure, which is encircled by and overlaps the base structure. The overlap is necessary to provide room for an emitter contact, but it causes another major problem with unwanted capacitance between the emitter and base structures. This capacitance slows down the operation of the HBT.
A dielectric layer is deposited over the emitter structure and is processed to form spacers around the emitter structure. An interlevel dielectric layer (ILD) is then deposited over the emitter and base structures.
Finally, contacts are formed in the ILD that connect with the collector, base, and emitter structures. Terminals are then connected to the contacts.
As previously mentioned, the emitter structure overlaps the base structure because it is necessary to provide room for the emitter contact to be formed. Since it is desirable to make the overlap as small as possible, it is desirable to have the emitter structure as small as possible. However, variations in the size of the emitter contact lead to a further major problem causing performance variations in the HBT from device to device.
Although the use of compound semiconductive materials has proven useful in HBTs, once formed by existing methods, this material is subsequently subjected to multiple thermal cycles, implantations and/or etching processes during the formation process of the remaining elements of the HBT, such as the deposition and etching of oxide layers, nitride layers and subsequently deposited polysilicon layers. Several of these processes inherently damage the compound semiconductive material. Etching polysilicon over a compound semiconductive layer, for example, adversely affects the compound semiconductive material because the etchants used do not selectively etch only the polysilicon. Some of the compound semiconductive material is also etched during this processing, resulting in HBTs that exhibit relatively poor noise performance and in leakage current, which in turn reduces integrated circuit yield.
One attempt to overcome the above mentioned problems involves selective epitaxial growth of the compound semiconductive material only over the active region of the HBT to form a self-aligned epitaxial intrinsic base structure. Selective epitaxy also may be used in a self-aligned emitter-to-base process in which an emitter window is defined by growing an in situ doped epitaxial layer over a patterned thin oxide/nitride pad.
In one method for fabricating a self-aligned double-polysilicon HBT using selective epitaxy, the intrinsic base is grown in the silicon substrate only in the active region of the silicon substrate. A polysilicon layer heavily doped with a dopant of a conductivity type opposite that of the substrate is deposited over the active region of the semiconductor substrate having a given conductivity type to form the emitter of the HBT.
For example, an n−-doped silicon substrate would have p-doped monosilicon layers deposited thereon. This monosilicon layer then has one or more compound semiconductive layers epitaxially grown over it. These layers are then covered with an upper insulating layer, for example silicon dioxide, to form a stack above the active region of the HBT. The epitaxial silicon layers are intended to eventually form the base structure of the HBT. The stack is then etched to define an emitter window. Electrically insulating regions or “reverse spacers” are separately made on the sidewalls of the emitter window. Next, polysilicon is deposited in the emitter window to form the emitter structure. The emitter structure is thus insulated from the extrinsic base structure by the reverse spacers and also by a portion of the upper insulating layer of the stack on which the emitter structure partially rests. This results in a more consistently small-sized emitter structure.
The adverse effects of etching the emitter window still persist however. During the operation of etching the stack, over-etching still occurs. The lack of adequate controls and reproducibility of over-etching typically results in the intrinsic base being thinned down or roughened. Thinning down of the compound semiconductive layer changes its performance resulting in integrated circuit failures or yield loss. Also, the roughness created on the compound semiconductive layer results in leakage current which either increases the power consumption or results in integrated circuit yield loss.
Furthermore, to improve the operating speed of a HBT, it is important that the base structure be thin enough to minimize the time it takes electronic charges to move from the emitter to the collector, thereby minimizing the response time of the HBT. It is also important, however, that the base structure have a high concentration of dopant in order to minimize base resistance. Typically, ion implantation techniques are used to form a base layer. However, this technique has the problem of ion channeling, which limits the minimum thickness of the base layer. Another disadvantage of ion implantation is that the compound semiconductive layer is often damaged by the ions during implantation.
Additionally, high-temperature annealing typically is required to drive dopants into the various material layers. This annealing process, however, alters the profile of concentration levels of the dopants within the various layers of semiconductive materials forming the transistor to create undesirable dopant profiles within the various material layers.
Existing methods of manufacturing HBTs still have the problems associated with over-etching, the detrimental effects of ion implantation and annealing, and consistency during manufacturing. HBTs still exhibit relatively high base resistance and poor noise performance.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.